Sulfate and acid resistant concrete and mortar

ABSTRACT

The present invention relates to concrete, mortar and other hardenable mixtures comprising cement and fly ash for use in construction and other applications, which hardenable mixtures demonstrate significant levels of acid and sulfate resistance while maintaining acceptable compressive strength properties. The acid and sulfate hardenable mixtures of the invention containing fly ash comprise cementitious materials and a fine aggregate. The cementitous materials may comprise fly ash as well as cement. The fine aggregate may comprise fly ash as well as sand. The total amount of fly ash in the hardenable mixture ranges from about 60% to about 120% of the total amount of cement, by weight, whether the fly ash is included as a cementious material, fine aggregate, or an additive, or any combination of the foregoing. In specific examples, mortar containing 50% fly ash and 50% cement in cementitious materials demonstrated superior properties of corrosion resistance.

The research leading to the present invention was conducted withGovernment support under Contract No. DE-FG22-90PC90299 awarded by theDepartment of Energy. The Government has certain rights in thisinvention.

CONTINUING INFORMATION

This application is a National Phase entry of the PCT/US95/06336, filedMay 19, 1995 which is a continuation-in-part of U.S. Ser. No. 08/246,861filed May 20, 1994, and now abandoned.

FIELD OF THE INVENTION

The present invention relates to concrete, mortar and other hardenablemixtures comprising cement and fly ash for use in construction and otherapplications, which hardenable mixtures demonstrate significant levelsof acid and sulfate resistance while maintaining acceptable compressivestrength properties.

BACKGROUND OF THE INVENTION Acid and Sulfate-Mediated Corrosion ofConcrete and Mortar

Concrete and mortar contain calcium hydroxide (Ca(OH)₂), which readilyreacts with acids or sulfates to form ettringite. This results inreduced strength of the concrete and mortar, which manifests as surfacedamage, and eventually leads to complete structural deterioration.Nowhere are these problems more acute than in our cities, wherebuildings and roadways slowly deteriorate under the assault of acid rainand other pollutants.

Corrosion of conventional concrete due to chemical attack of bothconcrete and the steel reinforcement costs an enormous amount of moneyannually for repairs and maintenance of structures. Sulfate and acidattack are a major problem with the durability of concrete. For pHvalues between 3 to 6, acid attack progresses at a rate proportional tothe square root of time (Neville, 1983, Properties of Concrete, 3rd. Ed,Pitman Publishing Inc.: London). Severe damage to concrete pipes insewer systems results from the action of the bacterium Thiobacillusconcreteavor, especially in warm climates. Sulfur-reducing bacteriareduce the sulfate present in natural water to produce hydrogen sulfideas a waste product. Another group of bacteria takes the reduced sulfurand oxidizes it back to sulfuric acid (Thornton, 1978, ACI J.Proceedings 75:577-584). Thus attack from sulfuric acid occurs,gradually dissolving and deteriorating concrete surfaces. This processis commonly known as "crown corrosion" in sewage collection systems.

In cement formulations, one way to minimize damage from acid or sulfateattack is to reduce the amount of C₃ A (tricalcium aluminate, 3CaO.Al₂O₃) present in the concrete. Such sulfate resistant cement is known asstandard portland cement type V. Type V portland cement specifies a C₃ Acontent of not more than 5%. Typically, however, the cost of standardportland cement type V is higher than standard portland cement type I.

Other strategies to increase the corrosion resistance of concrete, suchas polymer concrete, are also extremely expensive. Unfortunately, theexpense of making acid resistant concrete can outweigh the benefit to begained from using such concrete.

Another possible way to increase acid resistance is to introduce fly ashinto the concrete or mortar. Nasser and Lai (1990, Proceedings of theFirst Materials Engineering Congress, Denver, Colorado, pp. 688-97) andIrassar and Batic (1989, Cement and concrete Res. 19:194-202) reportedthat Class F fly ash was a good source of pozzolan, which could improveresistance of concrete to sulfate attack. The data on corrosionresistance of concrete samples monitored for more than three yearsindicated that concrete samples with 20% of cement replaced by fly ashprotected the steel reinforcement bars from corrosion better than plainconcrete (Maslehuddin et al, 1987, ACI J. Proceedings 84:42-50). Theresults of another study suggested that fly ash of finer particle sizehad greater resistance to sulfate attack (Sheu et al., 1990, SymposiumProceedings, Fly Ash and Coal Conversion By-Products: Characterization,Utilization and Disposal VI, Material Research Soc. 178:159-166).

However, the studies reported to date have not clearly revealed thedegree of corrosion resistance or indicated the exact characteristics ofcement or mortar containing fly ash. Partly, this was due to use ofgeneric fly ash, which tends to be of uncertain quality from one lot toanother. Without determining these characteristics, it is impossible toform any definite conclusions about the usefulness of concrete ormortar, much less risk using unpredictable materials on a constructionproject.

Fly Ash

Fly ash, a by-product of coal burning power plant, is produced worldwidein large quantities each year. In 1988, approximately 84 million tons ofcoal ash were produced in the U.S. in the form of fly ash (60.7%),bottom ash (16.7%), boiler slag (5.9%), and flue gas desulfurization(16.7%) (Tyson, 1990, Coal Combustion By-Product Utilization Seminar,Pittsburgh, 15 pp.). Out of the approximately 50 million tons of fly ashgenerated annually, only about 10 percent is used in concrete (ACICommittee 226, 1987, "Use of Fly Ash In Concrete," ACI 226.3R-87, ACI J.Proceedings 84:381-409). The remaining portion is mostly disposed of aswaste in landfills.

It is generally more beneficial for a utility to sell its ash, even atlow or subsidized prices, rather than to dispose of it in a landfill.Sales not only generate some income, but also, and more importantly,avoid the disposal cost. In the 1960's and 70's the cost of ash disposalwas typically less than $1.00 per ton. However, due to the morestringent environmental regulations starting in the late 1970's, thecost of ash disposal has rapidly increased from $2.00 to $5.00 per tonand is still rising higher (Bahor and Golden, 1984, Proceedings, 2ndInternational Conference on Ash Technology and Marketing, London, pp.133-136). The shortage of landfill due to environmental concerns hasfurther escalated the disposal cost. The Environmental Protection Agency(EPA) estimated in 1987 that the total cost of waste disposal at coalfired power plants ranged from $11.00 to $20.00 per ton for fly ash andbottom ash (Courst, 1991, Proceedings: 9th Int'l Ash Use Symposium,1:21-1 to 21-10).

This increasing trend of disposal cost has caused many concerns andresearchers are urgently seeking means for better utilization of flyash. One potential outlet for fly ash is incorporation in concrete ormortar mixtures.

Fly ash is used in concrete in two distinct ways, one as a replacementfor cement and the other as a filler. The first use takes advantage ofthe pozzolan properties of fly ash, which, when it reacts with lime orcalcium hydroxide, can enhance the strength of cementitious composites.However, fly ash is relatively inert and the increase in compressivestrength can take up to 90 to 180 days to materialize. Also, since flyash is just a by-product, the quality of fly ash has always been a majorconcern to the end users in the concrete industry.

Incorporation of fly ash in concrete improves workability and therebyreduces the water requirement with respect to the conventional concrete.This is most beneficial where concrete is pumped into place. Amongnumerous other beneficial effects are reduced bleeding, reducedsegregation, reduced permeability, increased plasticity, lowered heat ofhydration, and increases setting times (ACI Committee 226, 1987, supra).The slump is higher when fly ash is used (Ukita et al., 1989, SP-l 14,American Concrete Institute, Detroit, pp.219-240).

However, the use of fly ash in concrete has many drawbacks. For example,addition of fly ash to concrete results in a product with low airentrainment and low early strength development.

Thus, there is a need in the art for acid and sulfate resistant concreteand mortar.

There is a more urgent need for acid and sulfate resistant concrete andmortar at a reasonable cost, without sacrificing the rate of strengthgain specifications required for construction.

There is a further need in the art to find economical uses for fly ashproduced during combustion of coal.

These and other needs in the art are addressed by the instant invention.

The citation or identification of any reference in this applicationshall not be construed as an admission that such reference is availableas prior art to the present invention.

SUMMARY OF THE INVENTION

The present invention is directed to a hardenable mixture containing flyash with enhanced resistance to acid or sulfate attack. The hardenablemixture of the invention comprises cementitious materials and a fineaggregate, and may further comprise coarse aggregate. The cementitiousmaterials may comprise fly ash as well as cement. The fine aggregatecomprises sand, and may also comprise fly ash. The total amount of flyash in the hardenable mixture ranges from about 60% to about 120% of thetotal amount of cement, by weight; preferably from about 70% to about120%, and most preferably about 100%. According to the invention, thefly ash is fractionated by size or volume into fractions having anarrower range of particle sizes or volumes; preferably fractions havingfiner particle sizes or volumes are used.

Preferably, the mixtures of the invention are prepared with cementitiousmaterials comprising from about 5% to about 35% fly ash, more preferablyfrom about 10% to about 25% fly ash; and the fine aggregate comprisingsand and fly ash, such that the total amount of fly ash present in thehardenable mixture is about 60% to about 120% of the total amount ofcement in the cementitious materials in the mixture, by weight.

In a more preferred aspect of the invention, the fly ash has a finenessdefined by a fineness modulus of less than about 600, wherein thefineness modulus is calculated as the sum of the percent of fly ashretained on sieves of 0, 1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150, and 300microns. More preferably, the fly ash is a finer fraction of fly ashhaving a fineness modulus of less than about 350. The role of fly ashfineness in compressive strength gain of hardenable mixtures is morefully elaborated in copending U.S. application Ser. No. 08/246,875,filed May 20, 1994, Attorney Docket No. 715-1-035, entitled "IMPROVEDCOMPRESSIVE STRENGTH OF CONCRETE AND MORTAR CONTAINING FLY ASH" by thesame inventors as the inventors named herein, which is incorporatedherein by reference in its entirety. Use of finer fractions of fly ashis critical to preparing hardenable mixtures that provide maximumprotection from acid and sulfate attack, and demonstrate satisfactorycompressive strength properties for use in construction or otherapplications.

In a further aspect, the total amount of fly ash present in the mixtureis about 70% to about 110% of the total amount of cement in thecementitious materials in the mixture, by weight. Most preferably, thetotal amount of fly ash present in the mixture is about the same (100%)as the total amount of cement present in the cementitious materials inthe mixture, by weight.

In one aspect, the hardenable mixture is concrete. For example, theinvention is directed to a concrete comprising about 1 part by weightcementitious materials, about 1 to about 3 parts by weight fineaggregate, about 1 to about 5 parts by weight coarse aggregate, andabout 0.28 to about 0.6 parts by weight water, wherein the cementitiousmaterials may comprise fly ash as well as cement, wherein the totalamount of fly ash in the mixture ranges from about 60% to about 120% ofthe total amount of cement. Preferably, the fly ash has a fineness asdefined above.

In another aspect, the hardenable mixture is mortar. Accordingly, theinvention is directed to a mortar comprising about 1 part by weightcementitious materials, about 1 to about 3 parts by weight fineaggregate, and about 0.28 to about 0.6 parts by weight water, whereinthe cementitious materials may comprise fly ash as well as cement,wherein the total amount of fly ash in the mixture ranges from about 60%to about 120% of the total amount of cement. Preferably, the fly ash hasa fineness as defined above.

The present invention contemplates achieving the goals of acid andsulfate resistance and maximum compressive strength with the greatesteconomy. In one aspect, the invention provides hardenable mixtures thatcontain fly ash, in particular concrete or mortar, with greatercompressive strength than the equivalent cement or mortar composition,i.e., with the same amount of cement, lacking fly ash. This can beachieved by adding fly ash as an additive or fine aggregate substitute,or both, to a conventional mixture, without reducing the amount ofcement. Thus, the pozzolanic activity of the fly ash will increase thestrength of the hardenable mixture beyond that possible from the cementalone. Furthermore, the rate of strength gain will be very fast, sinceearly strength gain is provided by cement, and later strength comes withpozzolanic activity of the fly ash. Preferably, the total amount of flyash used as an additive is about equal to the total amount of cement, byweight. This embodiment of the invention is preferred for constructionprojects, where compressive strength gain is critical for maintaining aconstruction schedule, and where protection from corrosion is desirable.Although this embodiment of the invention costs more, for roughly thesame price as conventional hardenable mixtures, e.g., mortar or concretecontaining only cement, the present invention advantageously provides amuch stronger product that is acid and sulfate resistant.

In another embodiment, a hardenable mixture containing fly ash thatdemonstrates the same rate of compressive strength gain as the samemixture without fly ash, but at a cost savings, can be prepared. The flyash can be used as a partial replacement for cement in the cementitiousmaterials, e.g., to replace from about 5% to about 35% of cement. Inthis embodiment, the degree to which fly ash can replace cement withoutdecreasing the rate of compressive strength gain depends on the finenessof the fly ash; the greater the fineness of the fly ash, the greater theamount of fly ash that can be used to replace cement in the cementitiousmaterials. More fly ash can be included as fine aggregate or an additive(although the fly ash has pozzolanic activity whether introduced as acementitious material, a fine aggregate, or an additive). Preferably,the total amount of fly ash is equal to the total amount of cement.

In yet another embodiment, the invention provides for an inexpensivehardenable mixture, in which about 50% of the cement in cementitiousmaterials is replaced with fly ash. Preferably the fly ash is of a highdegree of fineness. Although the rate of compressive strength gain ofthis mixture would be much too slow for use in construction, after 180days or so the compressive strength of such a mixture is about the sameas a mixture without fly ash. Thus, concrete products, such as concretesewer pipes, that do not require immediate use can be prepared veryinexpensively.

In the best mode contemplated by the inventors for practicing theinvention, fly ash is used as a replacement for 25% of the cement incementitious materials in a hardenable mixture, e.g., concrete ormortar. More fly ash is used as an additive, or a replacement for fineaggregate, or both, so that the total amount of fly ash present in thehardenable mixture is about the same as the total amount of cementpresent in the hardenable mixture. Such a mixture provides all theadvantages of acid and sulfate resistance conferred by the presentinvention, with satisfactory compressive strength properties.

Accordingly, it is an object of the present invention to providehardenable mixtures that are highly acid and sulfate resistant, and thatdemonstrate greater compressive strength.

It is another object of the present invention to provide hardenablemixtures that are acid and sulfate resistant and less expensive, butwhich demonstrate the same properties of compressive strength.

It is yet another object of the present invention to provide hardenablemixtures that are acid and sulfate resistant, very inexpensive, and thatachieve the required compressive strength.

Still another object of the invention is to utilize fly ash.

These and other objects of the present invention can be readilyappreciated by reference to the following figures and detaileddescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents graphs showing the size distribution of fractionated flyash particles and cement particles (inverted triangles, 98% of whichhave a diameter of 75 μ or less). (A) Dry bottom boiler fly ash (solidsquare, in which 92% of the particles have a diameter of 75 μ or less)and fractions 1C (solid triangle, 95% less than 150 μ), 11F (soliddiamond, 96% less than 30 μ), 10F (open square, 94% less than 20 μ), 6F(open diamond, 99% less than 15 μ), 5F (X, 98% less than 10 μ), and 3F(open triangle, 90% less than 5 μ). (B) Wet bottom boiler fly ash (opensquare, 95% less than 75 μ) and fractions 18C (open triangle, 90.2% lessthan 75 μ), 18F (X, 100% less than 30 μ), 16F (open diamond, 99% lessthan 20 μ), 15F (99% less than 15 μ), 14F (solid diamond, 100% less than10 μ) and 13F (solid square, 93% less than 5 μ). Fly ash from dry or wetbottom boilers was collected and fractionated into six different sizedistribution fractions as described in the Examples, infra.

FIG. 2 is a graph showing the relationship between the weight of fly ashmortar samples and immersion time in a 100 ml/l H₂ SO₄ bath for samplescontaining 25% fly ash in cementitious materials. Plussign--non-fractionated dry bottom fly ash; opendiamond--non-fractionated wet bottom fly ash; opentriangle--fractionated dry bottom fly ash sample 6F; opensquare--control sample (no fly ash); X--fractionated dry bottom fly ashsample 16F25.

FIG. 3 is a graph showing the relationship between the weight of fly ashmortar samples and immersion time in a 100 ml/l H₂ SO₄ bath for samplescontaining 50% fly ash in cementitious materials. Symbols used are thesame as for FIG. 3.

FIG. 4 is a photograph of samples that had been immersed in a 100 ml/lH₂ SO₄ bath for 30 days. It is evident that the control (CF) and 20% flyash replacement samples (16F25, 6F25, MO25, and HO25) were severelycorroded by the treatment, but that the 50% replacement samples (16F50,6F50, MO50, and HO50) were relatively unaffected.

FIG. 5 is a graph showing weight loss and compressive strength loss ofmortar samples containing varying percentages of fly ash as cementitiousmaterials after acid bath treatment for 28 days. The data demonstratethat the optimum ratio of fly ash to cement for acid and sulfateresistance is 1:1. This ratio had the least weight loss and leastcompressive strength loss of all samples tested.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention relates to acid and sulfateresistant hardenable mixtures comprising fly ash. Preferably, the flyash used is of a defined degree of fineness.

Throughout this specification, where specific ratios, percentages, orproportions are mentioned, they are determined by weight and not byvolume.

The present invention is based, in part, on the observation thatregardless of the source mand chemical composition of fly ash, thepozzolanic properties of the fly ash depend on the degree of fineness ofthe fly ash. It has been surprisingly found that fractionation of flyash into fractions of a defined fineness modulus as herein definedprovides a high degree of quality control, regardless of theclassification or combustion conditions of the fly ash.

In specific embodiments, the corrosion resistance of fly ash mortar wasinvestigated using fly ashes of well defined physical and chemicalcharacteristics. Fly ash was introduced as a pozzolan into mortar toreact with calcium hydroxide in the mortar, thus reducing the reactivityof the mortar with acid. Fly ash mortar specimens made of differentpercentages of fractionated fly ash, but containing a normal amount ofcement, were immersed in a concentrated sulfuric acid bath to evaluatetheir resistance to acid attack. Strength and weight loss due to acidattack were monitored.

The invention is therefore based, in part, on the observation thatmortar containing fly ash was much more resistant to degradation by asulfuric acid bath. The optimum fly ash concentration for maximum acidresistance was found to be the same as the amount of cement, i.e., equalamounts of cement and fly ash in mortar gave maximum acid resistance.Similar results were obtained with concrete containing fly ash.

As used herein, the term "fly ash" refers to a solid material having achemical composition similar to or the same as the composition of thematerial that is produced during the combustion of powdered coal. In aspecific aspect, the solid material is the material remaining after thecombustion of powdered coal. ACI Committee 116 (1990, ACI 116-85, ACIManual of Concrete Practice Part I, American Concrete Institute,Detroit) defines fly ash as "the finely divided residue resulting fromthe combustion of ground or powder coal which is transported form thefirebox through the flue gases", and the term "fly ash" as used hereinencompasses this definition. Generally, fly ash derived from variouscoals have differences in chemical composition, but the principalcomponents of fly ash are SiO₂ (25% to 60%), Al₂ O₃ (10% to 30%), andFe₂ O₃ (5% to 25%). The MgO content of fly ash is generally not greaterthan 5%. Thus, the term fly ash generally refers to solid powderscomprising from about 25% to about 60% silica, from about 10% to about30% Al₂ O₃, from about 5% to about 25% Fe₂ O₃, from about 0% to about20% CaO, and from about 0% to about 5% MgO.

The term "fly ash" further contemplates synthetic fly ash, which may beprepared to have the same performance characteristics as fly ash asdescribed herein.

Presently, fly ash is classified primarily in two groups: Class C andClass F, according to the ASTM C 618 (1990, supra). Class F is generallyproduced by burning anthracite or bituminous coal, and Class C resultsfrom sub-bituminous coal or lignite. Generally, the fly ash from thecombustion of sub-bituminous coals contains more CaO and less Fe₂ O₃than fly ash from bituminous coal (Berry and Malhotra, 1980, ACI J.Proceedings 77:59-73). Thus, the CaO content of the Class C fly ash isusually higher than 10%, with the sum of the oxides of SiO₂, Al₂ O₃ andFe₂ O₃ not less than 50%. For Class F fly ash the CaO content isnormally less than 10% and the sum of the above mentioned oxides is notless than 70%.

The glassy phase of fly ash depends essentially on the combustionconditions and type of boiler. Non-fractionated fly ash obtained fromdifferent boilers, such as dry bottom boilers or wet bottom boilers, hasbeen found to behave differently. Boilers that achieve highertemperature yield fly ash with a more developed or pronounced glassyphase.

Alternatively, combustion in the presence of a fluxing agent, whichreduces the fusion temperature of the fly ash, can also increase theglassy phase of fly ash produced by combustion for lower temperatureboilers. Compressive strength of a hardenable mixture containing fly ashmay depend in part on the glassy phase of the fly ash, so generally flyash produced for higher temperature boilers, or produced in the presenceof a fluxing agent, or both, may be preferred. However, as demonstratedherein, the fineness modulus is the most important parameter forcompressive strength, and fractionated fly ash from any source, with adefined fineness modulus, can be used according to the invention.

Although fly ash generally comes in a dry and finely divided form, inmany instances, due to weathering and transportation processes, fly ashbecomes wet and often forms lumps. Such fly ash can be less reactiveunless the lumps can be dispersed into fine particles.

Pozzolan, as defined by ASTM C 593 (1990, ASTM C 593-89, Annual Book ofASTM Standards, Vol. 04.02), is "a siliceous or alumino-siliceousmaterial that in itself possesses little or no cementitious value butthat in finely divided form and in the presence of moisture willchemically react with alkali and alkaline earth hydroxides at ordinarytemperatures to form or assist in forming compounds possessingcementitious properties."

The rate of compressive strength gain of containing concrete or mortarcontaining fly ash depends on the fineness modulus of fractionated flyash. As used herein, the term "fineness modulus" refers to a measure ofthe distribution of volumes of particles of fly ash or distribution ofparticle sizes of the fly ash. According to the present invention, thefineness modulus is a distribution analysis that is much moreinformative than a median diameter determination or total surface areadetermination.

Preferably, the fineness modulus is determined as the sum of thepercentage of fly ash remaining on each of a series of different sizedsieves. Accordingly, the term "fineness modulus" refers to a relativevalue, which can vary depending on the series of sieves chosen. Since,according to the instant invention, fly ash particles of smaller size ordiameter are preferred for use in hardenable mixtures, more accuratedeterminations of fineness modulus are available if a series of smallersieves are chosen. Preferably, the size of the sieves is predominantlybelow 10 μ, e.g., the sieves may be 0.5, 1, 2, 3, 4, 5, 6, 7, 8 and 10microns. In this instance, the preferred fineness modulus will be ahigher absolute number, reflective of the greater degree of accuracy ofdetermination of this value for the smaller diameter or smaller size flyash particles.

The pozzolanic reaction of fly ash in a hardenable mixture comprisingcement is the reaction between constituents of the fly ash and calciumhydroxide. It is generally assumed to take place on the surface of flyash particles, between silicates and aluminates from the glass phase ofthe fly ash and hydroxide ion in the pore solution (Plowman, 1984,Proceedings, 2nd Int'l Conference on Ash Technology and Marketing,London, pp. 437-443). However, as demonstrated in copending applicationSer. No. 08/246,875, filed May 20, 1994, Attorney Docket No. 715-1-035,the pozzolanic reactions of fly ash are dependent on the volume of thefly ash particles: the smaller the particle volume, the more rapidly itcompletes its reaction with the cement to contribute to compressivestrength. The rate of solubility and reactivity of these glassy phasesin different types of fly ash depends on the glassy phase of fly ash,which in turn depends on the combustion temperature of the boiler thatproduced the fly ash. In addition to the effect of combustion conditionson the glassy phase of fly ash, different fly ashes from one class canbehave differently, depending on the SiO₂, Al₂ O₃ and Fe₂ O₃ content,and other factors such as the particle size distribution and storageconditions of the ash (see Aitcin et al, 1986, "Comparitive Study of theCementitious Properties of Different Fly Ashes," in Fly Ash, SilicaFume, Slag, and Natural Pozzolans in Concrete, SP-91, American ConcreteInstitute, Detroit, pp. 91-113; Liskowitz et al., 1983, Final Report,Vol. 1, U.S. Department of Energy, Morgantown Energy Technology Center,August, 211 pp.).

During hydration, portland cement produces a surfeit of lime (CaO) thatis released to the pore spaces. It is the presence of this lime thatallows the reaction between the silica components in fly ash and calciumhydroxide to form additional calcium silicate hydrate C-S-H!. He et al.(1984, Cement and Concrete Research 14:505-511) showed that the contentof crystalline calcium hydroxide in the fly ash-portland cement pastesdecreases as a result of the addition of fly ash, most likely resultingfrom a reaction of calcium with alumina and silica from fly ash to formaddition C-S-H. This process stabilizes the concrete, reducespermeability and increases resistance to chemical attacks.

Although not intending to be limited to any particular theory orhypothesis, it is believed that the ability of fly ash particles tolocate in the pore spaces of a hardenable mixture such as concrete ormortar determines how effective the particles are in contributing tocompressive strength or reacting with reactive components in cement.Thus, it is preferable to use finer fractions of fly ash, since the porespace is more accessible to particles having smaller volume. However,the invention contemplates optimizing the fineness of a fraction of flyash for a particular application, and contemplates using fractions offly ash having a range of values of fineness modulus.

Fractionation of fly ash can be accomplished by any means known in theart. Preferably, fractionation proceeds with an air classifying system.In a specific embodiment, infra, a MICRO-SIZER air classifying systemwas used to fractionate fly ash in six different particle size ranges.In another embodiment, the fly ash can be fractionated by sieving. Forexample, a 45 μ or smaller sieve can be used to select for particles ofa defined maximum size. In a further embodiment, the fly ash can beground to a desired size or fineness. This method can increase the yieldof fly ash; preferably the grinding process yields acceptably uniformparticles and does not introduce metallic or other impurities from thegrinder.

The term "cement" as used herein refers to a powder comprising alumina,silica, lime, iron oxide and magnesia burned together in a kiln andfinely pulverized, which upon mixing with water binds or unites othermaterials present in the mixture in a hard mixture. Thus, the hardenablemixtures of the invention include cement. Generally, the term cementrefers to hydraulic cements such as, but not limited to, portlandcement, in particular portland type I, II, III, IV and V cements.

As used herein, the term "cementitious materials" refers to the portionof a hardenable mixture that provides for binding or uniting the othermaterials present in the mixture, and thus includes cement andpozzolanic fly ash. Fly ash can comprise from about 5% to about 60% ofthe cementitious materials in a hardenable mixture of the invention;preferably, fly ash comprises from about 10% to about 25% ofcementitious materials. The balance of cementitious materials willgenerally be cement, in particular portland cement. In a specificembodiment, infra, the hardenable mixtures of the invention comprisesportland type I cement. It should be noted that where fly ash is used toreplace less than 50% of cement as cementitious materials in ahardenable composition of the invention, additional fly ash can beincluded in the composition so that the amount of fly as is in thepreferred range of about 100% of amount of cement.

In a specific embodiment, the fly ash makes up from about 10% to about25% of the cementitious materials, and fly ash is used as fine aggregatein a ratio of from about 4:1 to about 1:1 to sand. Thus, in thisembodiment, fly ash is an additive in addition to a replacement ofcement, or a replacement of cement and fine aggregate, or both.

The term "concrete" refers to a hardenable mixture comprisingcementitious materials; a fine aggregate, such as sand; a coarseaggregate, such as but not limited to crushed limestone or crushedbasalt coarse aggregate; and water. Concrete of the invention furthercomprises fly ash having defined fineness; preferably the fly ash isfractionated. In specific embodiments, concrete of the inventioncomprises about 1 part by weight cementitious materials, about 1 toabout 3 parts by weight fine aggregate, about 1 to about 5 parts byweight coarse aggregate, and about 0.28 to about 0.6 parts by weightwater, such that the ratio of cementitious materials to water rangesfrom approximately 3:1 to 1.5:1; preferably, the ratio of cementitiousmaterials to water is about 2.2:1. In a specific embodiment, theconcrete comprises 1 part cementitious materials, 2 parts siliceousriver sand or Ottawa sand, 3 parts 3/8" crushed basalt coarse aggregate,and 0.5 parts water.

The term "mortar" refers to a hardenable mixture comprising cementitiousmaterials; a fine aggregate, such as sand and/or fly ash; and water.Mortar of the invention further comprises fly ash, preferably havingdefined fineness. In a further aspect, the fly ash is used as fineaggregate in a ratio of from about 4:1 to about 1:1 to sand. In yet afurther embodiment, the fly ash is an additive in addition to areplacement of cement, or a replacement of cement and fine aggregate.

In specific embodiments, mortar of the invention comprises about I partby weight cementitious materials, about 1 to about 3 parts by weightfine aggregate, and about 0.28 to about 6.0 parts by weight water, suchthat the ratio of cementitious materials to water is approximately 3:1to about 1.5:1. In a specific embodiment, the mortar comprises 1 partcementitious materials, 2.75 parts Ottawa sand, and 0.5 parts water.

As noted above, fly ash can be used as a fine aggregate in acid andsulfate resistant concrete or mortar, in addition to or in lieu of areplacement for cement. In either case, the fly ash pozzolanic activitywill contribute to the cementitious properties of the mixture. It hasbeen found that substituting fly ash for a conventional fine aggregate,such as sand, provides the advantages of acid and sulfate resistancewith increased compressive strength of the concrete or mortar. When flyash is used as a partial replacement for cement, and added as a fineaggregate, the resulting hardenable mixture can have compressivestrength properties comparable to or greater than cement alone becauseof the pozzolanic activity of the fly ash. When fly ash is used solelyas an additive, hardenable mixtures with greatly increase compressivestrength properties result. According to the invention, preferably finerfractions of fly ash are used.

According to the present invention, the hardenable mixture can furthercomprise one or more of the following: glass fiber; silica fume, whichis a by-product from the silicon metal industry usually consisting ofabout 96%-98% reactive SiO₂, and which generally comes in very fineparticle sizes of less than 1 micron; and superplasticizer, an expensivebut common additive for concrete used to decrease the water requirementfor mixing the concrete, such as DARACEM®-100 (W. R. Grace).

Addition of silica fume can enhance the early rate of strength gain of ahardenable mixture, and therefore may be a desirable component ofhardenable mixtures of the invention. The silica fume, which isreactive, can also tie up acid and sulfate reactive materials in thecement.

In a specific embodiment, a hardenable mixture of the invention may alsocontain glass fibers for reinforcement. The use of glass fibers inhardenable mixtures of the invention for reinforcement can be achievedbecause the fly ash, particularly finer fractions of fly ash, reactsmore readily than glass fibers with reactive components of the cement,e.g., Ca(OH)₂, thus preventing long term reaction of the glass fiberswith these reactive components, which would otherwise degrade the glassfibers. Thus, the present invention advantageously provides for acid andsulfate resistant concrete and mortar that has significantly enhancedtensile strength because glass fibers can also be protected. Asdiscussed above, the most inert hardenable mixtures result are thosethat contain approximately equal amounts of fly ash, or fly ash andsilica fume (as discussed below), and cement.

In another specific embodiment, a hardenable mixture of the inventionfurther comprises glass fibers, and silica fume. Silica fume reacts morereadily with reactive components of cement than the glass fibers, andthus can provide early desirable protection of the glass fibers fromdegradation as well as early compressive strength gains. Subsequently,the fly ash will react with such reactive components of the cement, thusprecluding early and late reactivity of glass fibers. As noted above,reaction of glass fibers with alkali and alkali earth compounds can leadto degradation of the glass fibers, and loss of tensile strength of thehardenable mixture.

Concrete beams of the invention with dimensions of 3"×6"×27" can be usedto evaluate the bending strength of fly ash concrete, e.g., using simplebeam with third-point loading. Preferably, such test procedures are inaccordance with ASTM C 78 (1990, ASTM C 78-84, Annual Book of ASTMStandards, Vol 04.02).

Chemical Composition Chemical Composition of Fractionated Fly Ashes

The chemical composition of fractionated fly ashes are shown in Table 1.Sample CEM is the cement sample used in this study. Samples DRY and WETare the fly ashes from the non-fractionated dry and wet bottom boilerashes, respectively. 3F is the finest fly ash sample of the dry bottomash and 13F is the finest sample of the wet bottom ash. The coarsest flyashes samples of dry and wet bottom ash are 1C and 18C, respectively.

Both wet and dry bottom fly ashes used herein were classified as Class Ffly ash according to ASTM C418 (1990, supra). Most of the fractionatedfly ashes varied slightly in the oxide composition with changes inparticle size. It has been reported that separation of Class F (highcalcium) fly ash into size fractions does not result in significantchemical, morphological or mineralogical specification between particles(Hemming and Berry, 1986, Symposium Proceedings, Fly Ash and CoalConversion By-Products: Characterization, Utilization and Disposal II,Material Research Society 65:91-130). The SiO₂ content tends to be lowerwhen the particle size is larger. Differences in chemical compositionsof the two fly ashes were observed in the SiO₂, Fe₂ O₃, and CaOcontents. Samples of the dry bottom fly ash were about 10% richer inSiO₂ than the wet bottom fly ash. The CaO content of the dry bottom flyash varied from 1.90% to 2.99%, while for wet bottom fly ash, the CaOvaried from 6.55% to 7.38%. Fe₂ O₃ content of wet bottom fly ash wasabout twice as high in wet bottom than dry bottom fly ash. The highestconcentration of FeO₃ of each type of fly ashes was observed in thecoarsest particle sizes, i.e., 1C and 18C. Chemical composition of thefly ashes is shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    Chemical Composition of Fractionated                                          Fly Ashes and Cement                                                          Chemical Composition (%)                                                      Sam LOI                                                                              SO.sub.3                                                                          SiO.sub.2                                                                         Al.sub.2 O.sub.2                                                                   Fe.sub.2 O.sub.3                                                                  CaO K.sub.2 O                                                                         MgO Na.sub.2 O                                __________________________________________________________________________    CEM 0.73                                                                             2.53                                                                              20.07                                                                             8.84 1.41                                                                              60.14                                                                             0.86                                                                              2.49                                                                              0.28                                      3F0 4.97                                                                             1.69                                                                              49.89                                                                             26.94                                                                              5.43                                                                              2.99                                                                              1.76                                                                              0.99                                                                              0.33                                      5F  4.10                                                                             1.53                                                                              50.27                                                                             26.74                                                                              5.30                                                                              2.95                                                                              1.74                                                                              0.93                                                                              0.33                                      6F  3.12                                                                             1.09                                                                              51.40                                                                             26.54                                                                              4.91                                                                              2.72                                                                              1.71                                                                              0.74                                                                              0.31                                      10F 2.52                                                                             0.72                                                                              51.98                                                                             26.23                                                                              4.44                                                                              2.28                                                                              1.60                                                                              0.54                                                                              0.29                                      11F 2.04                                                                             0.53                                                                              51.27                                                                             26.28                                                                              4.42                                                                              2.02                                                                              1.55                                                                              0.49                                                                              0.26                                      1C  1.46                                                                             0.39                                                                              53.01                                                                             26.50                                                                              5.66                                                                              1.90                                                                              1.61                                                                              0.56                                                                              0.24                                      DRY 2.75                                                                             0.98                                                                              52.25                                                                             26.72                                                                              5.43                                                                              2.41                                                                              1.67                                                                              0.69                                                                              0.28                                      13F 2.67                                                                             3.81                                                                              38.93                                                                             24.91                                                                              12.89                                                                             6.85                                                                              2.10                                                                              1.55                                                                              1.31                                      14F 1.94                                                                             3.47                                                                              39.72                                                                             25.08                                                                              13.02                                                                             6.71                                                                              2.11                                                                              1.50                                                                              1.31                                      15F 1.88                                                                             3.33                                                                              40.25                                                                             25.02                                                                              13.12                                                                             6.60                                                                              2.11                                                                              1.47                                                                              1.30                                      16F 2.06                                                                             3.05                                                                              40.65                                                                             24.92                                                                              13.26                                                                             6.55                                                                              2.09                                                                              1.41                                                                              1.26                                      18F 1.94                                                                             2.94                                                                              41.56                                                                             24.47                                                                              14.21                                                                             6.58                                                                              2.01                                                                              1.40                                                                              1.17                                      18C 2.55                                                                             2.40                                                                              43.25                                                                             23.31                                                                              17.19                                                                             7.38                                                                              2.00                                                                              1.30                                                                              0.88                                      WET 2.05                                                                             3.13                                                                              41.54                                                                             24.74                                                                              14.83                                                                             6.89                                                                              2.07                                                                              1.43                                                                              1.17                                      __________________________________________________________________________

It is interesting to note that after fly ash was fractionated intodifferent sizes, loss of ignition (LOI) of the finest particle washigher than for larger particles. In other words, the LOI contentgradually decreased as the particle size increased. Ravina (1980, Cementand Concrete Research 10:573-80) also reported that the finest particleof fly ashes has the highest LOI values. Ukita et al. (1989, Fly Ash,Silica Fume, Slag, and Natural Pozzolans In Concrete, SP-114, AmericanConcrete Institute, Detroit, pp. 219-40) also showed that althoughchemical composition did not change when the median diameter of fly ashdecreased from 17.6 microns to 3.3 microns, LOI increased from 2.78 to4.37.

Our observations and these prior reports conflict with the report of ACICommittee 226 (1987, "Use of Fly Ash In Concrete," ACI 226.3R-87, ACI J.Proceedings 84:381-409) and of Sheu et al. (1990, Symposium Proceedings,Fly Ash and Coal Conversion By-Products: Characterization, Utilizationand Disposal VI, Materials Research Society 178:159-166), which statethat the coarse fraction of fly ash usually has a higher LOI than thefine fraction.

Particle Size Analysis of Fractionated Fly Ashes

The particle size distributions of fractionated fly ashes from the dryand wet bottom boilers are shown in FIGS. 1A and 1B, respectively. Thecurves for the original feed fly ashes are not as steep as others sincethe non-fractionated original feed ash includes the entire range ofsizes, and thus a wider range of size distributions than fractionatedsamples.

The percentage of fly ash in each fraction having a size less than aparticular size is indicated in parentheses in each curve. For example,in case of the 3F fly ash, the finest of dry bottom fly ash, 3F (90%-5μm) means that 90% of the fly ash particles are smaller than 5 microns.

From the original feed, each type of fly ash was fractionated into sixranges. As shown in FIGS. 1A and 1B, the particle size of fly ash variedfrom 0-5.5 micron to 0-600 microns. The median diameters of 3F and 13Fwere 2.11 and 1.84 microns, respectively, while the median diameters ofthe coarsest particle size, 1C and 18C, were 39.45 and 29.23 microns,respectively. For wet bottom fly ash, 13F was the finest fraction and18C was the coarsest.

The non-fractionated wet bottom fly ash was found to be finer than thenon-fractionated dry bottom fly ash. The particle sizes ofnon-fractionated dry bottom fly ash varied from about 1 micron to 600microns, with a median particle diameter of 13.73 microns. Thenon-fractionated wet bottom fly ash included particles up to 300 micronswith a median diameter of 6.41 microns. Particles from the smaller sizefractions tended to have a more spherical shapes (Hemming and Berry,1986, supra).

Fineness of Fractionated Fly Ash

Traditional values of fineness of fly ashes were determined both by wetsieve analysis and by the Blaine fineness together with the specificgravity of fly ashes, which are shown in Table 1. Median diameter, thediameter of which 50 percent of particles are larger than this size, isalso presented in this table. According to ASTM C-618 (1990, supra),specifications, fractionated 1C fly ash is unacceptable for use inconcrete since the percentage of the fly ash retained on sieve No. 325is higher than 34%.

                  TABLE 2                                                         ______________________________________                                        Fineness of Cement and Fractionated Fly Ashes                                         Specific Fineness:                                                            Gravity  Retained 45                                                                             Fineness:                                                                              Median                                    Sample No.                                                                            (g/cm.sup.3)                                                                           μm (%) Blaine (cm.sup.2 /g)                                                                   Diameter (μm)                          ______________________________________                                        CEM     3.12     --        3815     --                                        3F      2.54     0         7844     2.11                                      5F      2.53     0         6919     2.66                                      6F      2.49     0         4478     5.66                                      10F     2.42     0         2028     12.12                                     11F     2.40     1.0       1744     15.69                                     1C      2.28     42.0      1079     39.45                                     DRY     2.34     20.0      3235     13.73                                     13F     2.75     0         11241    1.84                                      14F     2.73     0         9106     2.50                                      15F     2.64     0         7471     3.09                                      16F     2.61     0         5171     5.54                                      18F     2.51     0         3216     9.84                                      18C     2.42     29.0      1760     29.25                                     WET     2.50     10.0      5017     6.41                                      ______________________________________                                    

Two methods were used to measure the fineness of fractionated fly ashes.The first method involved determining the residue on a 45 micron (No.325) sieve. Using the sieve No. 325 method, the fractionated fly ashsamples 3F, SF, 6F, 10F, 13F, 14F, 15F, 16F and 18F had the samefineness; all of them have zero retention. The second method was thesurface area measurement by air permeability test.

It can be noted from Table 3 that the finer the particle size offractionated fly ashes was, the higher the specific gravity and theBlaine fineness. In general, fly ash of greater fineness had greaterspecific gravity, in agreement with previous investigation (Hansson,1989, Symposium Proceedings, Fly Ash and Coal Conversion By-Products:Characterization, Utilization and Disposal V, Material Research Society136:175-183).

Density of fly ash from different electric generating plants varies from1.97 to 2.89 glcm³ but normally ranges between about 2.2 to 2.7 g/cm³(Lane and Best, 1982, supra). Work done by McLaren and Digiolin (1990,Coal Combustion and By-Product Utilization Seminar, Pittsburgh, p. 15)reported that Class F fly ash had a mean specific gravity value of 2.40.The specific gravity of fractionated fly ashes varies from 2.28 for thecoarsest fly ash to 2.54 for the finest fly ash for dry bottom fly ash,and from 2.22 for the coarsest to 2.75 for the finest wet bottom flyash.

The differences in density between fine-bottom and wet-bottom fly ashessuggest that the very fine particles of wet bottom fly ash arethick-walled, void free, or composed of more dense glasses andcrystalline components than dry bottom fly ash (Hemming and Berry, 1986,Symposium Proceedings, Fly Ash and Coal Conversion By-Products:Characterization, Utilization and Disposal II, Material Research Society65:91-103).

Corrosion Resistance of Fly Ash Mortar

Fractionated fly ashes 6F, 16F, the original feed of dry bottom fly ash(DRY), and wet bottom fly ash (WET) were mixed with cement to form flyash cement mortar. The mix proportions used are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Mix Proportion of Fly Ash Mortar                                              to Resist Acid Attack                                                                       Fly                                                             Sample                                                                              Cement  Ash     Sand  W/C + F)                                                                             Type of Fly Ash                            ______________________________________                                        CF    1.00    --      2.75  0.50   --                                         Dry25 0.75    0.25    2.75  0.50   Non-fractionated Dry                       Wet25 0.75    0.25    2.75  0.50   Non-fractionated Wet                       6F25  0.75    0.25    2.75  0.50   6F                                         16F25 0.75    0.25    2.75  0.50   16F                                        Dry50 0.50    0.50    2.75  0.50   Non-Fractionated Dry                       Wet50 0.50    0.50    2.75  0.50   Non-fractionated Wet                       6F50  0.50    0.50    2.75  0.50   6F                                         16F50 0.50    0.50    2.75  0.50   16F                                        ______________________________________                                    

As can be seen from Table 3, the percentage of fly ash used in the mixeswas 25 and 50 percent by weight of cementitious (cement and fly ash)materials. In other words, in this experiment, fly ash replaced cementin cementitious materials. The water to cementitious materials ratio ofall mixes was kept constant at 0.5.

Standard 2-inch cubes were cast and cured in saturated lime water about60 days before being put into the acid bath. Each cube was carefullyweighed. Fly ash cement mortar samples and control samples (100% cement,no fly ash, in cementitious materials), were then immersed in a 100 ml/1sulfuric acid (H₂ SO₄) bath. All samples were kept under the samecorrosive environment until the testing date. To evaluate the extent ofthe damage caused by acid attack, the samples were removed from the acidbath and washed with tap water. The samples were then weighed at thesaturated surface dry condition. The weight loss was then determined bycomparison with the weight of the original sample recorded earlier.

Results and Discussion

Sample designated "CF" is the control mix which contains no fly ash inthe mix. The number "25" and "50" stand for the percentage of cementreplaced by fly ash.

The weights of sample at different age after being submerged in theconcentrated 100 ml/l of H₂ SO₄ solution are tabulated in Table 4. Thecompressive strengths of fly ash mortars prior to being immersed in H₂SO₄ solution are also presented in Table 4.

                  TABLE 4                                                         ______________________________________                                        Effect of Fly Ash Cement Mortar                                               in H.sub.2 SO.sub.4 100 ml/l                                                  Weight at Different Ages (g)                                                  Sample                          14-  21-  30-  Comp                           No.    0-day  1-day   3-day                                                                              7-day                                                                              day  day  day  (psi)                          ______________________________________                                        CF     301.7   289.3. 262.2                                                                              206.5                                                                              139.5                                                                              100.1                                                                              69.9 9972                           DRY25  297.1  287.0   263.0                                                                              212.7                                                                              166.5                                                                              125.5                                                                              92.7 9121                           WET25  297.8  286.8   260.7                                                                              212.3                                                                              164.6                                                                              122.1                                                                              89.3 9250                           6F25   299.6  287.6   260.3                                                                              208.6                                                                              153.4                                                                              110.6                                                                              79.2 9415                           16F25  297.0  284.6   255.5                                                                              197.7                                                                              135.4                                                                              90.6 60.9 9311                           DRY50  295.8  295.4   293.6                                                                              289.5                                                                              280.1                                                                              276.8                                                                              257.8                                                                              5435                           WET50  291.9  291.8   291.3                                                                              291.1                                                                              291.3                                                                              276.8                                                                              233.5                                                                              6535                           6F50   294.8  297.7   294.8                                                                              293.6                                                                              294.3                                                                              292.6                                                                              287.2                                                                              5560                           16F50  298.3  298.2   298.0                                                                              298.2                                                                              298.5                                                                              290.8                                                                              269.3                                                                              6487                           ______________________________________                                    

For the control sample containing cement and no fly ash, the corrosiondue to acid attack is alarming. The weight losses of the control samplewas 30% at 7 days and 67% at 21 days. Such rapid deterioration of cementmortar is alarming. The data indicate that the free lime or calciumhydroxide in the cement control sample is extremely vulnerable to theacid attack.

Substitution of fly ash for cement may "tie up" free calcium hydroxidecompounds and prevent them from sulfuric acid attack. The resultspresented in Table 4 indicate that the 25% fly ash mortar samples werevulnerable to acid degradation, but slightly less than control sample.Partial protection from acid attack was observed regardless of the typeof fly ash or its particle size (FIG. 2).

When 50% of cement is substituted with fly ash in mortar, the extent ofweight loss was significantly reduced. After 7 days, there was nomeasurable weight loss; weight loss was limited to 6% after 21 days.With this degree of replacement, type of fly ash and its particle sizehad no significant effect on the corrosion resistance of fly ash mortar(FIG. 3).

After 30 days, particle size of fly ash demonstrates an effect on thecorrosion resistance (FIG. 4). The non-fractionated fly ash seemed tosustain more damage than the fractionated 15-micron ash samples (6FC50and 16FC50).

FIG. 4 shows the remains of the fly ash mortar samples after beingimmersed in the H₂ SO₄ for 30 days. Control and fly ash mortar sampleswith 25% replacement of cement with fly ash in the mix show severeweight loss after treatment in the 100 ml/l H₂ SO₄ solution. With 50percent fly ash in the mix, the mortar sample is much more resistant toacid attack than is the control and the 25 percent fly ash cementsamples.

In terms of compressive strength, the samples with 25% cementreplacement gave a higher compressive strength than the 50% one. Basedon the compressive strength, the samples can be divided into 2 groups.First is the control and the 25% fly ash samples, which demonstratevalues of compressive strength more than 9000 psi. The second groupconsists of the 50% fly ash mortar samples, which have strength betweenabout 5000 to 6500 psi.

The weight loss and compressive strength loss of mortar samplescontaining varying amounts of fly ash that replaces cement is shown inFIG. 5. This graph demonstrates a clear maximum of protection fromsulfuric acid attack when 50% of the cement is replaced with fly ash,i.e., when the amount of fly ash present is about the same as the amountof cement present in the mortar. The results further show that themaximum corrosion resistance is achieved when the ratio of cement to flyash is 1:1 in a mortar, or cement, composition. Higher or lower amountsof fly ash lead to increased weight loss and loss of compressivestrength from acid attack.

Clearly, compressive strength of a sample is not an accurate determinantof acid resistance; rather, it is the amount of fly ash-in the mix thatgoverns the resistance. These data indicate that the limit of fly ashcontent of cementitious materials, i.e., the maximum replacement ofcement with fly ash, to provide corrosion resistance against acid attackwhile maintaining an acceptable compressive strength, is about 35%.

The present invention is not to be limited in scope by the specificembodiments describe herein. Indeed, various modifications of theinvention in addition to those described herein will become apparent tothose skilled in the art from the foregoing description and theaccompanying figures. Such modifications are intended to fall within thescope of the appended claims. Various publications are cited herein, thedisclosures of which are incorporated by reference in their entireties.

What is claimed is:
 1. A hardenable mixture comprising cementitiousmaterials, fly ash, and a fine aggregate, wherein the cementitiousmaterials comprise fly ash as well as cement; and wherein the totalamount of fly ash ranges from about 60% to about 120% of the totalamount of cement in the hardenable mixture, by weight, wherein the flyash is characterized by at least 99% of the particles having a particlesize less than 20 microns and having a fineness modulus of less thanabout 600, wherein the fineness modulus is calculated as the sum of thepercent of fly ash particles having a size greater than 0, 1, 1.5, 2, 3,5, 10, 20, 45, 75, 150, and 300 microns.
 2. The hardenable mixture ofclaim 1, wherein the cementitious materials comprise from about 5% toabout 35% fly ash, and the fine aggregate comprises sand and fly ash. 3.The hardenable mixture of claim 2, wherein the cementitious materialcomprise about 25% fly ash, and the total amount of fly ash in themixture is about 100% of the total amount of cement, by weight.
 4. Thehardenable mixture of claim 1, wherein the total amount of fly ashpresent in the mixture is about 70% to about 110% of the total amount ofcement by weight.
 5. The hardenable mixture of claim 4, wherein thetotal amount of fly ash present in the mixture is about the same as thetotal amount of cement; by weight.
 6. The hardenable mixture of claim 1,wherein the fly ash is wet bottom fly ash having fineness modulus ofless than about
 350. 7. A concrete consisting of about 1 part by weightcementitious materials, about 1 to about 3 parts by weight fineaggregate comprising sand and fly ash, about 1 to about 5 parts byweight coarse aggregate, and about 0.28 to about 0.6 parts by weightwater, wherein the cementitious materials comprise from about 0% toabout 60% by weight fly ash and from about 40% to about 100% cement, andwherein the total amount of fly ash ranges from about 25% to about 150%of the total amount of cement, by weight, wherein the fly ash ischaracterized by at least 99% of the particles having a particle sizeless than 20 microns and having a fineness modulus of less than about600, wherein the fineness modulus is calculated as the sum of thepercent of fly ash particles having a size greater than 0, 1, 1, 5, 2,3, 5, 10, 20, 45, 75, 150, and 300 microns.
 8. The concrete of claim 7,wherein the cementitious materials comprise from about 5% to about 35%fly ash.
 9. The concrete of claim 8 wherein the cementitious materialcomprise about 25% fly ash, and the total amount of fly ash in theconcrete is about 100% of the total amount of cement, by weight.
 10. Theconcrete of claim 7, wherein the total amount of fly ash present in theconcrete is about 70% to about 110% of the total amount of cement, byweight.
 11. The concrete of claim 10 wherein the total amount of fly ashpresent in the concrete is about the same as the total amount of cement,by weight.
 12. The concrete of claim 7, wherein the fly ash is wetbottom fly ash having a fineness modulus of less than about
 350. 13. Amortar consisting of about 1 part by weight cementitious materials,about 1 to about 3 parts by weight fine aggregate comprising sand andfly ash, and about 0.28 to about 0.6 parts by weight water, wherein thecementitious materials comprise from about 0% to about 60% by weight flyash and about 40% to about 100% by weight cement; and wherein the totalamount of fly ash is about 25% to about 150% of the total amount ofcement by weight, wherein the fly ash is characterized by at least 99%of the particles having a particle size less than 20 microns and havinga fineness modulus of less than about 600, wherein the fineness modulusis calculated as the sum of the percent of fly ash particles having asize greater than 0, 1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150, and 300microns.
 14. The mortar of claim 13, wherein the cementitious materialscomprise from about 5% to about 35% fly ash.
 15. The mortar of claim 14,wherein the cementitious material comprise about 25% fly ash, and thetotal amount of fly ash in the mortar is about 100% of the total amountof cement, by weight.
 16. The mortar of claim 13, wherein the totalamount of fly ash present in the mortar is about 70% to about 110% ofthe total amount of cement, by weight.
 17. The mortar of claim 16,wherein the total amount of fly ash present in the mortar is about thesame as the total amount of cement, by weight.
 18. The mortar of claim13, wherein the fly ash is wet bottom fly ash having a fineness modulusof less than about 350.